1. Field of the Invention
This invention relates to inorganic oxide pigments and to a process for treating them to render them suitable for incorporation in paints or in other surface coating media.
2. Brief Description of the Prior Art
Different types of surface coating media present different pigment suitability requirements and, within limits, it is possible to tailor a pigment for a particular medium by surface coating the particles of the pigment with one or more inorganic oxides by, for example, precipitating the inorganic oxide onto the surface of the pigment. An improvement in one pigment property attained by the use of a particular combination of inorganic oxides in the surface coatings, or by a particular method of incorporating the inorganic oxide coating on the pigment, is often accompanied by a deterioration in other pigment properties. The water-dispersibility of a pigment which is a critically important criterion of suitability of a pigment intended for use in aqueous emulsion paints, depends on the isoelectric point of the pigment, that is, the pH at which the charge of the pigment particle is zero. For use in aqueous emulsion paints the pigment particles should give a slurry having as high a negative charge as possible and as high a pH as possible consistent with the limitation that at a pH above 10.5 any alumina present may dissolve. The lower the isoelectric point of the pigment the higher the negative charge of the slurry at a given alkaline pH. The isoelectric point, being a surface property of the pigment particles, may be controlled by surface coating the pigment particles with metal oxides. An alumina coating gives an isoelectric point at a pH of 9.2 and a titania coating gives an isoelectric point at a pH of 4.2 and a homogeneous mixed alumina/titania coating gives an isoelectric point at an intermediate pH depending on the proportion of alumina to titania. A substantial proportion of titania in a pigment surface coating can therefore improve dispersibility although this is accompanied by a deterioration in the case with which the pigment particles may be filtered and washed. This may be improved by increasing the proportion of alumina in the surface coating at the expense of decreased water dispersibility. A proportion of silica in the surface coating can also improve water dispersibility as can the use of a higher pH during precipitation of the inorganic oxide surface coating. Other examples of the problem of attaining a suitable combination of properties to render the pigment suitable for use in a paint medium are well known in the art and much effort has been expended in alleviating them.
It is now possible to obtain grades of pigment which perform adequately in a variety of different aqueous emulsion paints or in a variety of different organic-based paints. The problem of attaining a grade of pigment which performs adequately in both aqueous and organic media, the so-called "universal" pigment, is still with us and although several such grades are on the market none fully satisfies all the requirements for use in both types of medium.
Two fundamentally important properties of organic based paints are gloss and durability. Durability is, generally, improved by increasing the proportion of alumina in the inorganic oxide surface coating but with the previously discussed attendant deterioration in water dispersibility. The gloss properties of organic based paints can be impaired by increasing the relative proportion of silica in the inorganic oxide surface coating.